1. Field of the Invention
This invention relates to a liquid-crystal panel having insulated-gate thin-film transistors, and more particularly, to a structure of a terminal electrode for the gate electrode of the thin-film transistor.
2. Description of the Prior Art
By the use of amorphous silicon for the semiconductor layer which is combined with an anodic oxidation film to form the gate insulator film, it is possible to obtain a thin-film transistor (TFT) having excellent characteristics. In Japanese Laid-Open Patent Application No. 58-147069, the following characteristics for this structure have been described: (1) excellent insulating qualities (because are no pinholes) having superior reliability and high-voltage resistance; (2) small mobile-ion density; (3) small interface state density of the semiconductor; and (4) large field effects upon the semiconductor.
The process for the production of a liquid-crystal panel having such TFTs will be described hereinafter in detail with reference to the figures. FIG. 2 is a sectional view showing a basic structure of the TFT. A film of Ta is deposited on the top of the glass substrate 1, and gate electrodes 2a, gate bus lines 2b and terminal electrodes 3, are formed in a fixed pattern by a photolithography process (see FIG. 3). Then, by the photolithography process, only the gate electrodes 2a treated by anodic oxidation are exposed, and the glass substrate is immersed in an aqueous solution of ammonium tartrate for the anodic oxidation. The anodic oxidation forms a film of Ta.sub.2 O.sub.5 having a thickness of 1000 .ANG. by applying a constant voltage of 6.5 V. As a result, a gate electrode 2a of Ta and a first insulator film 4 that is made of a thin oxide film formed on the entire surface of the gate electrode 2a are obtained. On the top of the first insulator film 4, a second insulator film 5 is deposited to form a film of Si.sub.3 N.sub.4 having a thickness of 1000 .ANG. by CVD method or a sputtering method. For the second insulator film 5, it is possible to use SiO, SiO.sub.2, Y.sub.2 O.sub.3, Al.sub.2 O.sub.3, MgF.sub.2, or the like, instead of Si.sub.3 N.sub.4. The second insulator film 5 functions to protect the anodic oxidation film made of Ta.sub.2 O.sub.5 (that is, the first insulator film 4). The first insulator film 4 and the second insulator film 5 together form the gate insulator film. Next, for the semiconductor layer 6, a layer of amorphous silicon having a thickness of 3000 .ANG. is deposited by glow discharge, and then a layer of Ti having a thickness of 3000 .ANG. is deposited to form the source electrode 7 and the drain electrode 8, and the finished TFT results.
Thereafter, the transparent conductive coating 9 that is made of, for example, indium tin oxide (ITO) forms the picture elements. A layer of Si.sub.3 N.sub.4 having a thickness of 3000 .ANG. deposited by CVD method to form a protective film 10. The protective film 10 does not function only to protect the layer of amorphous silicon, but also depletes carriers in the vicinity of the surface of the semiconductor layer 6. Thereby the amount of leakage current in the "off" state is decreased, and the device characteristics of the TFT are greatly improved. Then, an aligning treatment is performed by a polyimide application and rubbing and a liquid-crystal panel is obtained by sealing and filling the device with liquid crystals (not shown).
Thereafter, flexible printed-circuit boards equipped with a driving LSI are connected to the terminal electrodes 3 on the top of the liquid-crystal panel by an anisotropic conductive sheet. The production of a liquid-crystal display module by this method will be explained below. FIG. 4 shows the planar structure of this liquid-crystal display module, and FIG. 5 shows a sectional view taken along line X-X' of FIG. 4. The flexible printed-circuit boards 22 equipped with a liquid-crystal driving LSI 21 are connected to the terminal electrodes on the top of the liquid-crystal panel 23 through an anisotropic conductive sheet that is positioned in the space between the printed-circuit boards 22 and the terminal electrodes. Also, a double-sided printed-circuit board 24 having input signal lines (common wiring) is attached to the opposite face of the flexible printed-circuit boards 22, and a liquid-crystal display module results.
In this case, the Ta of the terminal electrode 3 significantly deteriorates (that is, the electrode is oxidized) by the curing of the polyimide mentioned above (at 200.degree. C. for 2 hours), by the curing of the sealing resin (at 180.degree. C. for 2 hours), or a like process. Therefore, the connection to the flexible printed-circuit board by the anisotropic conductive sheet becomes unstable.